A variety of mount assemblies are presently available to isolate vibrations on automobile and truck engines and transmissions. One of the most popular mounts today is the hydraulic-elastomeric mount. An example of such a mount is shown in U.S. patent application Ser. No. 785,243, filed Oct. 7, 1985 and entitled "Hydraulic-Elastomeric Mount Displacement Decoupler".
The hydraulic mount assembly disclosed in this patent application includes a molded, hollow rubber body that is closed by a resilient diaphragm so as to form a cavity. This cavity is partitioned by a plate into two chambers that are in fluid communication through an orifice in the plate. A primary chamber is formed between the plate and the body and a secondary chamber is formed between the plate and the diaphragm.
A decoupler is mounted in the plate so as to reciprocate and thereby produce small volume changes in the two chambers. When, for example, the decoupler moves toward the diaphragm, the volume of the primary chamber increases and the volume of the secondary chamber decreases. In this way, at certain small vibratory amplitudes and high frequencies, fluid flow between the chambers through the orifice is avoided and undesirable hydraulic damping is eliminated. In effect, this decoupler is a passive tuning device that enhances the performance capability of the mount.
The orifice is formed to extend in a track around the perimeter of the orifice plate. Each end of the track has one opening, one communicating with the primary chamber and the other to the secondary chamber. The orifice track provides the hydraulic mount assembly with another passive tuning component, and when combined with the decoupler provides at least three distinct modes of operation that exhibit different dynamic characteristics. The operating mode is primarily determined by the flow of the fluid between the two chambers. Small amplitude vibrating inputs produce no damping due to decoupling, as described above. Secondly, large amplitude vibrating inputs from the engine produce high velocity fluid flow through the orifice track. The resulting high level of damping force and smoothing action creates another operation mode of the mount. Conversely, medium amplitude inputs produce lower velocity fluid flow in the mount resulting in a medium level of damping.
While the three distinct modes of operation provided by present hydraulic mounts thus provide generally satisfactory operation, they are not sufficient to furnish the desired damping and noise suppression under all the continuously varying conditions encountered during vehicle operation. More specifically, certain circumstances such as engine lugging, rough road conditions, sudden turning and/or rapid acceleration or deceleration produce vibrations of both varying frequency and amplitude that simply cannot be properly isolated by a mount assembly providing only limited modes of operation. A need is therefore identified for a hydraulic mount assembly that provides for an active or variable control of the dynamic characteristics. These dynamic characteristics of the mount can then be tuned, either manually or automatically, to provide the most effective and efficient damping and noise suppression over the entire range of expected operating conditions.